1. Field of the Invention
The present invention relates to an interferometer and a measurement method employing such an interferometer, and particularly to an interferometer for measuring length of end standards, such as a gauge block or a length bar.
2. Description of the Related Art
Conventional interferometers are employed for high precision measurement of length gauge blocks and length bars used for correcting various types of measurement machines. Such an interferometer operates by first calculating the integer of interference fringe N by preliminary measurement for measuring an end standard at a precision of half wavelength or less, obtaining the interference fringe fraction .epsilon. by main measurement using laser interference, and finally determining the dimension L of the end measure from the equation L=.lambda./2 (N+.epsilon.)
When measuring an end standard at a precision of half wavelength or less, a so-called coincidence method is used where a plurality of wavelengths are used to perform preliminary measurements for each wavelength and use the value which coincides independent of a particular wavelength. As an interferometer, a Twyman-Green interferometer, for example, is used.
However, even when the coincidence method is employed, a comparator separate from the laser interferometer must be employed nd separate measurement must be made, and this in turn requires or temperature equalizations (removing any temperature non-uniformity within a work [end measure] due to human body heat and environment temperature, etc. by allowing the object to rest work for a few hours after placement, the work being the object to be measured) for both preliminary and main measurements (the main measurement being a measurement using laser interferometer), making the measurement steps complicated and thus causing a problem of efficiency being low.
Moreover, when a high precision measurement is made, temperature compensation is required to take into account elongation of the end measure according to the coefficient of linear expansion. However, if the preliminary measurement and the main measurement using laser interference are made separately, it is possible for the error in the thermometer used for preliminary measurement and the error in the thermometer used for main measurement to differ, increasing the difficulty of accurately determining the integer of interference fringe N.